Efforts to recover the resources of the extensive Buttes Geothermal Field in Imperial County, Calif., have been made since the initial discovery of this field in 1958. The underground reservoir generally contains brine at temperatures above 500.degree. F. and at a pressure sufficient to maintain the brine in liquid phase. The geothermal brine is in chemical equilibrium with the surrounding reservoir rock and is unique in that it contains approximately 25 percent by weight dissolved solids, primarily chlorides, including various elements which are economically desirable to recover. Little well-to-well variation in chemical composition has been found -- typical samples of the geothermal brine have been analyzed and found to contain, on the average, the following solids dissolved therein:
______________________________________ Concentration in mg/l ______________________________________ Sodium 69,700 Potassium 16,400 Lithium 200 Calcium 30,600 Iron 1,400 Manganese 1,600 Lead 100 Magnesium 80 Silver nil Zinc 360 Copper 3 Barium 1,300 Strontium 580 Aluminum 95 Ammonium (NH.sub.4) 400 Fluoride 450 Chloride 148,400 Boron 180 Sulfate (SO.sub.4) 130 Silica (SiO.sub.2) 860 Total dissolved solids .about.272,900 ______________________________________
To produce the geothermal reservoir, a well is drilled into the reservoir, typically about 5000 feet in depth. The reservoir is normally in a state of thermal inversion and once this inversion is disturbed, resulting in alleviation of the formation pressure, steam forms in the well casing and entrains brine as it flows up the well. Typically, the produced fluid at the wellhead will be in the order of 90 percent steam by volume (20 percent by weight), the remainder being liquid brine at temperatures of 400.degree.-450.degree. F. and pressures of 200-450 psia.
Efforts to produce this brine commercially have been unsuccessful due to the formidable problems of scaling in the surface equipment, such scaling being chiefly due to the high concentration of dissolved silica in the brine. Analysis of samples of the geothermal brine from the Buttes Field indicates an average of about 860 mg/l silica in solution with a range between 600 to 1300 mg/l. The temperature dependence of the solubility of amorphous silica in water has been determined as:
______________________________________ Temperature Solubility of SiO.sub.2 .degree. F. (amorphous) mg/l ______________________________________ 500 1,300 400 950 300 600 200 330 100 140 ______________________________________
Thus, the average dissolved silica concentration in the produced brine is near, and sometimes above, the saturation level of the produced brine at wellhead temperatures. In order to handle the brine and process it for mineral and/or energy recovery, the temperature and pressure must be reduced. As a consequence the silica, having become supersaturated, comes out of solution. The resulting scale is of amorphous structure, containing certain amounts of the other dissolved solids, and is tightly adherent to the walls of surface piping and brine processing equipment. Desired rates of flow for commercial operation cannot be maintained with this scale build-up on system surfaces.
In addition, geothermal brine has a relatively low pH, ranging from about 4.8 to 5.3 but usually about 5.0, at wellhead conditions. Due to the existence of chloride ions and multiple valence ions in their reduced state, corrosion of the system surfaces has been a severe problem in areas where scaling does not occur. Thus, depending on location, the brine-handling system may either be subjected to undesired silica scale formation or subjected to severe corrosion.
It is the principal object of the invention to treat the produced brine whereby silica scale formation is either prevented or reduced to a degree where it may be removed periodically by conventional means, and to render the brine less corrosive, so that the brine may be handled with conventional piping and equipment. In this invention this object is achieved in a manner that renders the brine suitable for subsequent production of steam and/or desirable elements dissolved in the brine.